More particularly stated, the present invention is concerned with a combination direct and indirect evaporative heat exchange apparatus and method which achieves maximization of the heat exchange efficiencies of both the indirect and direct evaporative cooling sections.
In a direct evaporative heat exchanger, only an air stream and an evaporative liquid stream are involved and the two streams evaporatively exchange heat when they come into direct contact with each other; the evaporative liquid is typically water. In an indirect evaporative heat exchanger, three fluid streams are involved; an air stream, an evaporative liquid stream, and an enclosed fluid stream. The enclosed fluid stream first exchanges sensible heat with the evaporative liquid through indirect heat transfer, since it does not directly contact the evaporative liquid, and then the evaporative liquid and the air stream evaporatively exchange heat when they directly contact each other.
Closed loop evaporative heat exchangers can be broadly grouped into three general categories: 1) Sensible heat exchanger-direct evaporative heat exchanger systems where one of the fluid streams from the sensible heat exchanger is piped to a direct evaporative heat exchanger; 2) Stand alone indirect evaporative heat exchangers; and 3) Combination direct and indirect evaporative heat exchangers.
Shell and tube refrigerant condensers or sensible heat exchangers which are connected to separate cooling towers are examples of the first group and they represent the predominantly used heat exchange methods in which evaporative cooling is normally utilized. Products referred to as "coil sheds" are also part of this first group, and coil sheds consist of a cooling tower (direct evaporative heat exchanger) located directly above a non-ventilated coil section (sensible heat exchanger).
Stand alone indirect evaporative heat exchangers represent the next group and these devices are typically not as popular as those of the first group. The majority of evaporative condensers and evaporative fluid coolers are of this type. Products with the air and evaporative liquid streams in counterflow, crossflow or concurrent flow are commercially available, although the counterflow design predominates.
The last and currently least popular group involves products which combine both indirect and direct evaporative heat exchange sections. The present invention is part of that group and it represents a unique improvement over the prior art in this group by offering the most efficient way to construct closed loop evaporative heat exchangers.
When the invention is used as a closed circuit cooling apparatus, such as a closed loop cooling tower, an initially hot fluid, usually water, is generally directed upwardly through a series of circuits which comprise an indirect evaporative heat exchange section, where the hot water undergoes indirect sensible heat exchange with a counterflowing, cooler evaporative liquid gravitating over the outside surfaces of the circuits. In the preferred embodiment, the coldest water leaving each of the circuits is equally exposed to the coldest uniform temperature evaporative liquid and coldest uniform temperature ambient air streams available. This leads to a more uniform and necessarily more efficient method of heat transfer than accomplished by the prior art. As heat is transferred sensibly from the hot fluid, the evaporative liquid increases in temperature as it gravitates downwardly through the indirect evaporative heat exchange section. Simultaneously, cooler ambient air is drawn down over the circuits in a path that is concurrent with the gravitating evaporative liquid. Part of the heat absorbed by the evaporative liquid is transferred to the concurrently moving air stream, while the remainder of the absorbed heat results in an increase of temperature to the evaporative liquid as if flows downwardly over the circuits. The evaporative liquid then gravitates over a direct evaporative heat exchange section. The direct evaporative heat exchange section utilizes a separate source of cool ambient air to directly cool the now heated evaporative liquid through evaporative heat exchange. Air flow through the direct section is either crossflow or counterflow to the descending evaporative liquid. This now cooled evaporative liquid is then collected in a sump, resulting in a uniform temperature cooled evaporative liquid which is then redistributed to the top of the indirect evaporative section.
When applied as an evaporative condenser, the process is the same as explained for the closed circuit fluid cooling apparatus except that since the refrigerant condenses at an isothermal condition, the flow of the fluid, now a refrigerant gas, is typically reversed in order to facilitate drainage of the condensate.
When applied as a wet air cooler, either with an initially cold single phase fluid or an evaporating refrigerant the process is the same as explained earlier for the fluid cooling or condensing applications, respectively, except that the heat flows in the opposite direction.
Prior art combinations of direct and indirect evaporative heat exchange sections (U.S. Pat. Nos. 4,112,027, 4,683,101, and 3,141,308) teach us to place a crossflow direct evaporative section above the indirect section. However, the direct-over-indirect arrangement taught by either disclosure unfavorably results in a temperature gradient being formed in the cooling water as it descends through the direct evaporative section. The gradient forms when the crosscurrently flowing cooling air absorbs heat from the descending water, creating uneven heat exchange and resultant non-uniform temperature water along the longitudinal extent of the direct section. As discovered by U.S. Pat. No. 4,683,101, this gradient in water temperature can be in the order of 6.degree.-10.degree. F. across the depth of the direct section, and when the direct section is directly above the indirect section, the non-uniform temperature water drains directly onto the series of underlying circuits comprising the indirect section thereby creating non-uniform heat transfer from circuit to circuit. Those in the art know that the non-uniform heat transfer in this instance is a source of overall thermal inefficiency to the tower. Furthermore, the non-uniform heat transfer represents additional operational inefficiencies in a condensing application because liquid condensate will back up within the unevenly loaded circuits and limit the surface area available for condensing. The prior art U.S. Pat. No. 4,683,101 tried to address this problem by physically changing the orientation of the indirect heat exchange circuits, as well as the internal fluid flow direction within the circuits so that the hottest fluid to be cooled within the circuits was in thermal exchange with the hottest temperature of cooling water within the gradient. However, that arrangement failed to address the water temperature gradient problem itself and therefore, neglected the effects it had on heat exchange within the indirect heat exchange section.
In the closed circuit fluid cooling tower of the present invention, it was discovered that distributing an initially uniform temperature evaporative liquid over the indirect evaporative heat exchange section had a substantial effect upon the uniformity of heat exchange within that section. This invention also discovered that if the indirect heat exchange section no longer had the direct heat exchange section immediately lying above it, added advantages in cooling efficiency could be realized from various air and water flow schemes.